The majority of optical blood pulse measurements (e.g., pulse oximetry) are carried out nowadays utilizing transmissive measurement techniques requiring sensor devices that are mountable over two opposite sides of a thin body part/organ (e.g., finger tip or earlobe). Particularly, transmissive measurement techniques use a light source typically placed on one side of the body part/organ for irradiating light signals thereover, and a light detector placed on the other side of the body part/organ to measure the intensity of light signals passing through it.
However, there are various advantages for reflective blood measurement techniques, which are considered to be preferable in certain applications, particularly in types of wearable medical devices, such as medical watches, for example. Some of the advantages of reflective blood measurement techniques include, inter alia, the ability to conduct optical measurements on almost any part of the body, including thick organs. Other outstanding advantages are associated with the reduced energy consumption of reflective measurement techniques stemming from the minimal energy consumption required to reflect light from tissue layers, as opposed to transmissive techniques where the light is required to pass through the entire width of the organ.
There are however various limitations associated with the reflective measurement techniques, such as low signal-to-noise ratios (SNR), the low AC/DC ratios (Wherein AC is light waves reflected from (capillary) blood vessels, and DC is a combination of light waves reflected from other parts of the examine organ tissue and light waves reflected directly from the organ surface without passing through the examine tissue. Attempts to overcome these limitations of the reflective measurement techniques by increasing the power of the irradiated light (e.g., by increasing the electrical current supplied to the light source, and/or by increasing the number of light sources), typically also result in respective increase of noise components in the measured signals (due to respective increase of the baseline DC component), and thus do not provide satisfying results. There is thus a need to improve the quality of optical signals measured by reflective blood measurements techniques, to provide higher AC/DC ratios and improve the signal-to-noise ratios of the measured signals.
Some reflective measurements techniques known from the patent literature are briefly described herein below.
US Patent Publication No. 2009/082642 discloses a system and method for use in monitoring of biological parameters of a subject. The system includes an illumination unit including at least one light source of at least one pre-selected wavelength band, to be applied to a selected region in the subject; and a detection system configured for measuring reflections of the light at different angles and different spatial locations with respect to the illuminated region. The detection system is configured and operable to detect spatially separated light components corresponding to the specular dependent component of the signal and the pulsatile related diffused component of the signal coming from the subject in different directions respectively, thereby defining at least two independent channels of information, enabling identification of the reflected signal part dependent on motion effects.
US Patent Publication No. 2014/213917 describes a biofeedback device and a light sensor thereof that can be mounted on or integrated with eyewear such as swimming goggles. The biofeedback device may include a heart rate measurement apparatus comprising a reflected green light sensor, and first, second, and third green light emission elements. The biofeedback device may include a housing having a first portion and a second portion, which each of the first and second portions having a first side and a second side. At least a portion of the heart rate measurement apparatus may be disposed within the housing first portion and may be exposed through an opening in the second side of the housing first portion. The biofeedback device may also include an opening that allows the device to be removably engageable with at least a portion of the swimming goggles.
Chinese Paten publication No. 102198005 describes a reflective wrist oximeter with an electrocardiograph function, which comprises a blood oxygen acquisition module, an electrocardiosignal acquisition module, a processing module, a display module, a shell and wrist bands, wherein the blood oxygen acquisition module comprises a light-emitting driver, a light-emitting tube and a photosensor; the light-emitting tube and the photosensor are arranged at the same side of a position to be detected, and an included angle having preset degrees is formed between the light-emitting tube and the photosensor; the electrocardiosignal acquisition module comprises a first electrode and a second electrode and is used for acquiring electrocardiosignals; the processing module is used for processing pulse blood oxygen data acquired by the acquisition module and the electrocardiosignals acquired by the electrocardiosignal acquisition module; the display module is used for displaying information processed by the processing module to a user; the wrist bands are connected to both sides of the shell; the first electrode is arranged at one side of the shell, which is in contact with the wrist; the display module is arranged at the other side of the shell, which is opposite to the first electrode; the second electrode is arranged outside the display module; and the blood oxygen acquisition module is arranged inside or outside the second electrode. The reflective wrist oximeter delicately combines the reflective blood oxygen technology and the electrocardiograph technology, and is convenient to operate.